Interesting question. Personally I would rather win a test series in Australia than the World Cup. However that would be the selfish response. Reason being that were England to win the Cricket World Cup, it would no doubt go along way towards increasing the exposure of English cricket and thus further strengthening the youth coming through. The knock-on effect I believe would be greater from winning the world cup than winning a series in Australia.
However I believe a reasonable percentage of fans in England who are avid cricket fans would rather win a test series, especially in Australia, but the boost to English cricket from winning the world cup in terms of providing far more "part-time" fans would outweigh the boost from winning in Australia. It would however be mighty close, as has been seen by Englands summer Ashes win that has seen a record number of cricket equipment sold this year.
Hmm more I think about, I think both would be a massive boost to Englands domestic cricket, and I will go for the test series in Australia, as personally that is what I would rather watch